This invention relates to the recovery of liquid and gaseous products from subterranean oil shale formations and, more particularly, to the in situ production of shale oil from oil shale.
One technique for recovering shale oil is to form an in situ retort in a subterranean oil shale formation. The retort is in the form of a cavity in essentially undisturbed oil shale filled with fragmented oil shale. The cavity and bed of fragmented shale therein can be formed by mining and blasting. The fragmented oil shale at the top of the in situ retort is ignited to establish a combustion zone at the top of the in situ retort, and the combustion zone is advanced slowly down through the fragmented oil shale by passing gas downwardly through the in situ retort. The heat generated in the combustion zone is transferred the fragmented oil shale below the combustion zone to establish a retorting zone on the advancing side of the combustion zone wherein kerogen in the fragmented oil shale is converted to shale oil and other liquid and gaseous products. Thus, an elevated temperature retorting zone moves from the top to the bottom of the in situ retort in advance of the combustion zone, and the resulting shale oil passes to the bottom of the in situ retort for recovery.
In the described recovery process, it is desirable that the retorting zone move as a horizontal plane wave down through the in situ retort. If the combustion zone becomes significantly skewed, shale oil released in the lagging region of the retorting zone may be destroyed in the leading region of the combustion zone, thereby reducing the yield of the shale oil from the in situ retort. It is also important that the fragmented shale within the in situ retort present a low resistance to gas flow therethrough, otherwise the inlet gas introduced at the top of the in situ retort would be under high pressure which would consume an excessive amount of energy to compress the inlet gas sufficiently to move it from the top to the bottom of the in situ retort. The resistance to flow depends upon the void volume or void fraction of the in situ retort and the height of the in situ retort.